In the art of drilling wells for recovery of hydrocarbons, the process incorporates a drill string which has a plurality of threaded tubular members such as drill pipe being approximately 30 foot each in length, the drill pipe threaded end to end which is then used to rotate the drill bit either from the surface or through the use of a drill motor which would rotate the bit without the rotation of the drill pipe itself. Often times during that process, the drill string will become lodged at a certain point along its length within the borehole.
In the efforts to dislodge the drill pipe or other tools lodged downhole, a type of tool known as a jarring tool would be used in such an attempt. In the current state of the art, jarring tools may be utilized to either jar the stuck or the lodged portion of pipe either in the up or down direction, depending on the makeup of the tool. In most cases, it would be more desirable to jar down on the pipe than to jar up. The reason for this is that drill pipe will usually get lodged when it is being pulled up as opposed to being moved downward, so jarring downward will more likely free the pipe. In such a case, the pipe is probably wedged against an obstruction caused by the upper movement of the pipe, and jarring upward may tend to wedge the debris around the section of pipe even tighter.
Methods of downward jarring which are currently used in the art include applying compression on the drill string to which a down jar has been attached, whereby the jar releases at a pre-set load, allowing the hammer of the jar to freely travel a short distance impacting the anvil of the tool, delivering a downward blow. The effectiveness of this method has limitations, due to compressional buckling of the drill string, as well as drag. Therefore, it is often difficult to achieve a large downhole jarring force in a vertical well, and the problem is exacerbated in the horizontal portion of a directional drilling operation. A jar in the upward direction can be attached to the top of the stuck pipe or tool, and the jar can be pulled upward until it is tripped. While this type of jarring can produce more force than downward jarring, it is typically in the wrong direction for most instances of stuck pipe. Typically, in oilfield drilling operations, when a drill bit and/or drill string becomes stuck, a jar that is coupled to the drill string may be used to free the drill bit and/or the drill string. The jar is a device used downhole to deliver an impact load to another downhole component, especially when that component is stuck. There are two primary types of jars, hydraulic and mechanical. While their respective designs are different, their operation is similar. Energy is stored in the drillstring and suddenly released by the jar when it fires, thereby imparting an impact load to a downhole component. Jars may also be used to recover stuck drill string components during drilling or workover operations
Drilling jars typically have a sliding mandrel in a sleeve. In use, the mandrel is driven up or down by some form of stored energy, a hammer on the mandrel striking an anvil on the sleeve so as to impart a shock and (it is hoped) free the stuck pipe. One common form of drilling jar is a hydraulic jar. A hydraulic jar includes two reservoirs of hydraulic fluid separated by a valve. When tension or compression is applied to the tool in a cocked position, fluid from one chamber is compressed and passes through the valve at high flow resistance into the second chamber. This allows the tool to extend or contract. When the stroke reaches a certain point, the compressed fluid is allowed to suddenly bypass the valve. The jar trips as the fluid rushes into the second chamber, instantly equalizing pressure between the two chambers and allowing the hammer to strike the anvil. The greater the force on the jar, the sooner and more forceful the release.
As jars are returned to the surface after use and/or placed in a derrick, jars may accidentally fire. Such accidental firing can result in significant safety hazards at a drilling location. Traditionally, in an attempt to prevent accidental firing, an external jar clamp is manually placed on a shaft of the jar located between the internal mandrel assembly and the external cylinder assembly. The clamp acts as an external stop that would prevent axial movement of the tool. However, in the event the external clamp was not properly fastened to the jar, the clamp could fall off of the jar during storage, thereby creating a falling object hazard at the drilling location.
In certain situations, internal mechanical latches have also been used in an attempt to prevent accidental firing of the jar. However, internal mechanical latches result in additional steps prior to firing a jar, increasing operational complexity and may unlatch if a load is accidentally exceeded on the rig floor.
Accordingly, safety mechanisms for jars to prevent accidental firing may be desired.